With the rapid growth of data services, the pressure of data transmission on licensed carriers is also increasing. Therefore, using unlicensed carriers to share data traffic on licensed carriers becomes an important evolution direction of a Long Term Evolution (LTE) system in the future. Unlicensed carriers have the following features:
1. Unlicensed spectrums do not need to be purchased, i.e., no cost is required for such spectrum resources. Thus, unlicensed carriers are featured by being free/low-cost.
2. Both individuals and enterprises can participate in deployment. Equipment of equipment producers can be deployed at will. Thus, unlicensed carriers are featured by low access requirements and low costs.
3. Unlicensed frequency bands such as 5 GHz and 2.4 GHz can all be used. Thus, unlicensed carriers are featured by a large available bandwidth.
4. Unlicensed carriers are featured by shared resources. That is, when multiple different systems or different operators of the same system are operating in unlicensed carriers, ways of resource sharing may be considered to improve the spectral efficiency.
5. Unlicensed carriers are featured by multiple radio access technologies. That is, different communication standards are involved, collaboration is difficult and network topologies are diversified.
6. Unlicensed carriers are featured by multiple radio access sites. That is, the number of users is large, collaboration difficulty is large and overheads of centralized management are large.
7. Unlicensed carriers are featured by multiple applications. That is, multiple services are mentioned to be operatable in unlicensed carriers, such as Machine to Machine (M2M) and Vehicle to Vehicle (V2V).
It can be seen that if the LTE system can use existing unlicensed carriers, potential spectrum resources of the LTE system will be improved greatly, allowing the LTE system to have lower spectrum costs.
However, before using unlicensed carriers, devices such as a base station or a user equipment (UE) in the LTE system have to meet regulatory requirements of unlicensed carriers. That is, before using unlicensed carriers to transmit information, the devices have to implement a listen-before-talk (LBT) mechanism. Furthermore, due to the progress of the 3rd Generation Partnership Project (3GPP) Licensed Assisted Access (LAA) conference, for an uplink now, only basic uplink functions are required to ensure that the uplink works normally. Basic functions of the uplink include: an Uplink (UL) LBT process before data transmission, a sounding reference signal (SRS) for measuring an uplink channel, a physical random access channel (PRACH) and the like. For SRS transmission, SRS is mainly used to maintain uplink timing and achieve uplink and downlink scheduling (due to channel reciprocity) of the base station. Therefore, if uplink transmission is allowed, several issues need to be considered: first, how to execute LBT before uplink transmission so as to successfully transmit a SRS; second, after the LBT is executed successfully, how to transmit the SRS; and third, if the LBT and the SRS coexist in the same orthogonal frequency division multiplexing (OFDM) symbol, how to address the problem of collision between the LBT and the SRS. That is, only when the above problems are solved can uplink transmission timing be ensured, can selective scheduling of uplink frequencies (resources with good channel conditions are allocated to uplink transmission) of the base station be ensured, and can determination of Modulation and Coding Scheme (MCS) be ensured.